What Is SIP Calling? A UK Guide to SIP, VoIP and the PSTN Switch-Off

SIP Calling Article Summary

1. SIP calling uses the Session Initiation Protocol to establish and manage voice, video, and other communication sessions over IP networks.
2. The UK PSTN transition makes SIP and VoIP particularly relevant for businesses preparing their phone systems for the move to digital services.
3. SIP-based communications can improve flexibility, scalability, and mobility, while reliable connectivity, emergency calling, security, and resilience remain important parts of deployment.

SIP calling is becoming increasingly relevant for UK businesses as traditional telephone infrastructure gives way to digital services. It provides a flexible way to establish and manage business communications over IP networks, while supporting the cloud-based phone systems, remote working models, and unified communication tools that many organisations already use.

The timing matters. BT plans to retire the legacy Public Switched Telephone Network (PSTN) used across the Openreach network by 31 January 2027, with businesses moving towards digital alternatives such as VoIP-based services [1]. As of July 2026, around 350,000 business premises were still connected to the old network [2].

For businesses reviewing their telephone infrastructure, understanding where SIP fits into that transition is increasingly useful.

What Is SIP?

SIP stands for Session Initiation Protocol. It is an application-layer signalling protocol used to create, modify, and terminate communication sessions between participants [3].

These sessions can include Internet telephone calls, video conversations, multimedia conferences, and other real-time communications.

One distinction is particularly important: SIP manages the signalling around a communication session rather than carrying the voice or video itself.

In a typical Internet call, SIP can establish the connection, locate participants, negotiate which media will be used, modify the session if required, and end it when the conversation finishes. Other protocols generally transport the actual audio or video.

This makes SIP one component within a broader IP communications environment.

For a broader introduction to Internet telephony, see Ringover's guide to VoIP.

What Is a SIP Call?

A SIP call is a communication session established and controlled using the SIP protocol.

The participants may be using desk phones, computers, mobile devices, softphones, or other compatible endpoints. The communication travels through an IP network, which may be a private company network or the public Internet.

Although people often associate SIP exclusively with voice calls, the protocol can establish sessions involving several types of media [3].

A SIP-based communication could therefore involve:

• A business voice call
• A video conversation
• A conference session
• A combination of voice and video
• Communication between several participants
• Integration between Internet-based and conventional telephone networks

The actual capabilities available to users depend on the phone system and provider rather than SIP alone.

Why SIP Calling Matters for UK Businesses in 2026

For UK organisations, SIP has become more relevant because the underlying telephone infrastructure is changing.

BT plans to retire its legacy PSTN network on 31 January 2027 [1]. Businesses using lines delivered through the Openreach network therefore need to move towards digital services.

Other operators with their own networks may follow different migration schedules, so organisations should confirm the timetable directly with their communications provider.

The change involves more than office desk phones.

Legacy telephone connections may still support equipment such as:

• Payment terminals
• Lift emergency phones
• Security alarms
• Entry systems
• Fax machines
• Building-management equipment
• Other specialised services historically connected to analogue telephone lines

Hundreds of thousands of UK business premises were still connected to the legacy network in mid-2026 [2].

This creates a useful opportunity to review the broader communications environment rather than simply replacing one line with another.

A business can consider how calls are routed, which employees need access from different locations, how business applications connect with telephony, and what resilience arrangements are required once communications depend more heavily on digital infrastructure.

What Is the Difference Between a SIP Phone and VoIP/an IP Phone?

The terms SIP phone, VoIP, and IP phone are closely related, although they refer to slightly different concepts.

An IP phone is a telephone that communicates through a Voice over Internet Protocol network rather than relying entirely on a traditional analogue telephone line.

A SIP phone is a type of IP phone that uses SIP as its signalling protocol. In other words, SIP phones fall within the broader category of IP phones. Both can take several forms.

Hardware IP and SIP phones

A hardware-based IP phone often looks similar to a traditional office desk phone. Instead of connecting to a conventional analogue telephone socket, it connects to an IP network, typically through Ethernet or Wi-Fi.

Some desk phones support several SIP accounts or lines.

Software phones

A softphone provides telephone functionality through software rather than a dedicated handset.

Employees can make and receive calls through an application on a:

• Desktop computer
• Laptop
• Smartphone
• Tablet

This approach can be particularly useful for remote and hybrid teams because employees can access business telephony without remaining tied to a particular desk.

SIP Calling vs. Traditional Landlines

Traditional landline services historically relied on the PSTN and circuit-switched telephone infrastructure.

SIP takes a different approach. Signalling takes place across IP networks, allowing communication systems to operate through Internet-connected infrastructure.

A SIP-based call may remain entirely within an IP environment.

For example, two users communicating through compatible SIP endpoints can establish a session without the call travelling through the traditional PSTN.

When a SIP user calls a conventional telephone number on another telephone network, the provider can use a gateway to connect the IP-based environment with the destination network.

This distinction becomes increasingly relevant as UK businesses move away from legacy PSTN services.

Table: SIP Calling vs. Traditional Landline Telephony

How SIP Telephony Works

How Do You Make a SIP Call?

A SIP call can be made using several types of compatible device.

One option is a physical SIP desk phone connected to your company network.

Another is a softphone installed on a computer, smartphone, or tablet. Many modern cloud phone providers supply their own applications, allowing employees to access business calling without configuring SIP manually.

A typical setup involves:

1. A compatible phone, application, or communication platform
2. A reliable Internet or private IP connection
3. A SIP account or managed communications service
4. Appropriate user credentials and configuration
5. Audio equipment such as a handset, headset, microphone, or speakers

The exact process depends on the service provider.

Managed business phone systems often hide much of the underlying SIP configuration from employees. Users simply sign into the application and make calls through a familiar interface.

For mobile teams, Ringover provides iOS and Android applications alongside desktop access.

Technical Explanation of a SIP Call

A simplified SIP call can be broken into several stages.

1. The endpoint becomes available

A SIP device or application can register its location with the relevant SIP infrastructure. This allows the system to determine where requests for that user should be directed.

2. The caller starts a session

When a user makes a call, the originating application sends a SIP INVITE request.

The request contains information required to begin negotiating the communication session [3].

3. The destination is located

SIP servers can help determine where the intended recipient can currently be reached and route the request accordingly.

This means an identity can be associated with different endpoints rather than a single physical telephone socket.

4. The session is negotiated

The endpoints agree on suitable session characteristics, such as the media and codecs they will use.

5. The conversation begins

Once the session has been accepted, the audio or video media is generally transmitted using a separate real-time media protocol.

SIP continues to manage the session rather than carrying the audio stream itself.

6. The session can change

SIP can support changes while the conversation is in progress, depending on the system.

For example, participants or media characteristics may change.

7. The call ends

When one participant ends the conversation, a SIP BYE request can terminate the session [3].

What Is a SIP Trunk?

A SIP trunk is a digital connection that allows a business telephone system to send and receive communications through a SIP-enabled service provider.

It can effectively replace groups of traditional physical telephone lines for organisations using compatible PBX infrastructure.

A SIP trunk may connect:

• A business IP PBX
• A cloud communications environment
• A service provider's network
• Public telephone networks
• Remote offices or users

For companies with an existing IP PBX, SIP trunking can therefore offer one route towards digital telephony without replacing every element of the current system at once.

Cloud-native businesses may take a different approach and use a fully managed cloud phone system where the underlying SIP infrastructure is largely invisible to users.

What Are the Benefits of SIP Calling?

SIP can provide several practical advantages when used as part of a modern IP communications environment.

Lower Telephony and Infrastructure Costs

Internet-based business phone systems can reduce reliance on traditional on-premise telephone infrastructure.

VoIP has long been associated with greater flexibility and potential cost efficiencies compared with conventional circuit-switched telephony [4].

Savings depend on the organisation's existing infrastructure, provider, calling destinations, hardware requirements, and service plan.

For some businesses, the largest benefit comes from reducing the amount of dedicated hardware and separate communication services they need to maintain.

Greater Scalability

IP-based telephony can make it easier to add employees, numbers, devices, or locations as an organisation grows.

A traditional telephone installation may require additional physical lines or PBX capacity. A cloud-based service can often provision additional users through software.

This can be particularly useful for:

• Growing sales teams
• Seasonal customer-service operations
• Recruitment agencies
• Multi-site companies
• Hybrid teams
• Businesses opening new locations

More Flexible Working

SIP and VoIP allow business communications to operate across Internet-connected devices.

Employees may therefore be able to use their business number from the office, home, or another suitable location without relying on the same physical handset.

This supports remote and hybrid working arrangements while helping organisations maintain consistent business identities.

Unified Communications

SIP can form part of a wider unified communications environment.

Calls can sit alongside other business communication functions such as messaging, video meetings, CRM activity, call recording, and analytics.

Employees can therefore work with fewer disconnected systems and preserve more context between customer interactions.

Business Application Integrations

Modern cloud phone systems frequently connect with business software such as:

• CRMs
• Help desks
• Applicant tracking systems
• Collaboration platforms
• Productivity tools

These integrations can reduce repetitive data entry and make call activity easier to associate with the relevant customer, prospect, or candidate.

Call Quality

SIP-based voice calls can provide high-quality audio when the underlying network is properly configured.

Call quality depends on factors including:

• Available bandwidth
• Latency
• Jitter
• Packet loss
• Wi-Fi or Ethernet quality
• Network congestion
• Provider infrastructure
• Quality-of-Service configuration

VoIP therefore benefits from thoughtful network planning rather than relying on Internet availability alone [4].

SIP Calling, 999/112, and Business Continuity

Emergency calling deserves particular attention when moving from traditional telephone lines to IP-based services.

UK regulated voice providers are required to take appropriate measures to provide access to emergency organisations, and accurate and reliable caller-location information must be made available for 999 and 112 calls where technically feasible [5].

Businesses should confirm how their chosen service handles:

• 999 and 112 calls
• Caller-location information
• Remote and home-based employees
• Multiple offices
• Number portability
• Temporary work locations

Location can be more complex in an IP environment because an employee may take the same application or account to another physical location.

That makes provider configuration and internal procedures important.

Plan for Power and Internet Outages

Traditional analogue landline phones could often continue operating during a local power failure because the line itself supplied power.

Digital phone services generally rely on powered networking equipment. During a power cut, routers, switches, handsets, and other equipment may stop working unless suitable backup arrangements are in place [1].

For business environments, resilience planning may include:

• Uninterruptible power supplies
• Backup Internet connections
• Mobile connectivity
• Call forwarding
• Secondary office locations
• Business-continuity procedures
• Alternative methods for reaching emergency services

The appropriate level of resilience depends on how critical telephony is to the organisation.

A small professional-services company and a 24-hour healthcare facility, for example, will have very different requirements.

How Do You Get Started With SIP Calling?

Moving to SIP or another digital business phone solution works best as a planned migration rather than a simple equipment swap.

1. Audit your existing telephone environment

Document your:

• Telephone numbers
• Users
• Desk phones
• PBX
• Call-routing rules
• IVR
• Hunt groups
• Call recording
• Fax services
• Alarms
• Payment terminals
• Lift phones
• Entry systems
• Other devices connected to telephone lines

The UK PSTN transition makes this step especially important because legacy lines may support services that employees rarely associate with the phone system [2].

2. Decide between SIP trunking and cloud telephony

Businesses with an established compatible IP PBX may prefer SIP trunking.

Companies seeking simpler management, remote access, built-in integrations, and reduced on-site infrastructure may prefer a fully cloud-based business phone system.

3. Review your network

Check Internet capacity, network design, Wi-Fi coverage, latency, and redundancy.

Voice traffic is sensitive to network conditions, so a reliable data connection plays an important role in call quality.

4. Choose devices

Your team may use:

• SIP desk phones
• Existing compatible IP phones
• Computer softphones
• Mobile applications
• Headsets

Some organisations combine several of these options.

5. Plan number migration

Confirm which existing telephone numbers can be ported to the new provider and when the port will take place.

Businesses may also choose to activate new UK or international business numbers as their requirements change.

6. Confirm emergency calling

Review 999 and 112 support, caller-location procedures, remote-working policies, and outage arrangements before deployment [5].

7. Test before full deployment

Run calls across several scenarios:

• Internal calls
• External outbound calls
• Inbound calls
• Transfers
• Call forwarding
• Voicemail
• Mobile calls
• Conference calls
• Call recording
• Emergency-call procedures
• Internet or power disruption scenarios

A pilot group can help identify configuration issues before the system reaches the wider organisation.

SIP and BYOD Security

SIP- and VoIP-based communications can work well with Bring Your Own Device policies because employees can access business communication services through software on mobile devices.

That flexibility also changes the security model.

Personally owned devices can introduce additional considerations around access control, device configuration, authentication, business data, software updates, and loss or theft.

VoIP environments should therefore be incorporated into the organisation's wider cybersecurity programme [4].

Companies using mobile and personal devices should consider:

• Strong authentication
• Mobile-device management where appropriate
• Access controls
• Encryption
• Software updates
• Clear BYOD policies
• Remote access procedures
• Processes for lost or stolen devices

The convenience of IP-based communications works best when accompanied by equally modern security practices.

What Should You Look for in a SIP or Cloud Phone Provider?

SIP support alone tells you relatively little about the day-to-day quality of a business communications service.

During evaluation, consider the complete operating environment.

Connectivity and reliability

Ask how the provider designs redundancy, monitors service availability, and responds to incidents.

Call quality

Understand the codecs supported and the network conditions required for consistent audio.

Emergency calling

Confirm arrangements for 999 and 112 calls and how caller-location information is handled.

Security

Review encryption, authentication, administrator permissions, access controls, and account-management procedures.

Business continuity

Understand what happens during broadband, power, device, or service interruptions.

Number porting

Confirm which UK and international numbers can be transferred and how migrations are managed.

Integrations

Check whether the platform connects with the CRM, help desk, ATS, or productivity applications your employees already use.

Features

Consider the capabilities employees actually require, including:

• IVR
• Call queues
• Transfers
• Recording
• Call forwarding
• Analytics
• Mobile access
• Conference calling

Support

Check onboarding, documentation, migration assistance, technical support, and service hours.

The Benefits of Unified Communications and SIP for UK Businesses

Moving towards Internet-based communications can allow businesses to rethink how employees communicate rather than simply recreating the old phone system in digital form.

Centralisation can bring calling, customer information, routing, recording, and other tools closer together.

That may provide practical improvements in several areas.

Simpler administration

Administrators can often manage users, numbers, call routing, and permissions through software rather than arranging physical changes to telephone lines.

Greater mobility

Employees can work across several devices and locations while retaining their business communication tools.

Easier scaling

New users and locations can often be added without reproducing a traditional PBX installation.

More connected customer information

CRM and help-desk integrations can associate communication activity with the appropriate contact or account.

More predictable communications management

Bringing several functions under one platform can reduce the number of separate systems teams need to manage.

SIP is only one piece of this picture, but it has played an important role in enabling modern IP-based communications.

Looking for a Modern Business Phone System? Ringover Can Help

Businesses preparing for the move towards digital telephony may prefer a cloud phone system that manages much of the underlying technical infrastructure for them.

Ringover's business phone system combines Internet-based business calling with communication and management features designed for modern teams.

Employees can make and receive calls through desktop and mobile applications, helping them stay connected in the office, at home, or while travelling.

Ringover also includes business telephony features such as:

• Call forwarding
• Conference calls
• Call recording
• Call routing
• IVR
• Business numbers
• Call logs and analytics
• CRM integrations

Companies operating across several markets can also activate international numbers, while virtual numbers allow business calls to reach Internet-connected applications rather than a single physical telephone.

For UK organisations reviewing their communications ahead of the PSTN transition, this provides an opportunity to move towards a more flexible cloud-based setup while bringing calls and business applications into a more connected environment.

Contact Ringover to discuss your current business phone setup and explore your options for moving towards cloud telephony.

Citations

• [1]https://www.ofcom.org.uk/phones-and-broadband/landline-phones/future-of-landline-calls
• [2]https://www.openreach.com/news/openreach-warns-businesses-just-six-months-left-before-the-uks-old-phone-network-is-switched-off/
• [3]https://www.rfc-editor.org/rfc/rfc3261.html
• [4]https://www.nist.gov/publications/security-considerations-voice-over-ip-systems
• [5]https://www.ofcom.org.uk/phones-and-broadband/telecoms-infrastructure/compliance-programme-into-access-to-emergency-services

Published on September 17, 2020.